At present, in automotive heat exchangers as represented by a radiator, a condenser, an intercooler and the like, and other heat exchangers, heat sinks and the like which are manufactured using aluminum alloys, a brazing method using a non-corrosive fluoride-based flux under inert gas atmosphere or a brazing method using a brazing filler metal including about 0.5 to 1.5% by mass of Mg under vacuum atmosphere are predominantly adopted.
In many cases of brazing using the above-mentioned flux, a brazing object member is put into a desired assembled state after being processed by press molding or the like, a turbid solution prepared by dissolving flux powder in a solvent is applied to the assembled body followed by drying, and brazing is then performed in a non-oxidizing atmosphere by high-purity nitrogen gas. This method has a problem in that even the use of flux, or the installation or management of an application step thereof is costly.
Furthermore, it is known that the flux is partially evaporated in the process of brazing heating, and adhered and accumulated on a inside wall of a furnace, and periodic maintenance of the furnace for removal of deposits is also produced as a necessary cost. In accordance with the current promotion of automotive lightening, further reduction in thickness and increase in strength of material are demanded even in the automotive heat exchangers. Although addition of Mg to aluminum alloy is very effective for increasing the strength of aluminum material, flux-using brazing have a problem in that all the added Mg cannot be contributed to the increase in strength, since MgF2 having high melting point is produced through reaction of Mg with the flux, and this inhibits the brazing or consumes Mg in the material. Namely, in the flux brazing, the addition of Mg cannot be actively adopted as a means for increasing the strength of the material, under present circumstances, due to restrictions on the adding portion or amount of Mg in a product. In inverter coolers or the like for use in hybrid cars or electric cars, recently, the use of flux can be limited for reasons such that flux residue itself inhibits the solderability of a semiconductor component or the like.
On the other hand, in vacuum brazing, Mg added to brazing filler metal evaporates from the material in a temperature-raising process for brazing, and breaks an oxide film on aluminum material surface that is a brazing inhibiting factor in doing so, and, in the atmosphere, it brings the furnace internal atmosphere into a brazable state by its getter effect of coupling with moisture or oxygen therein. Although this method needs no flux application step, it has a problem in that considerable costs are produced for a vacuum furnace that is expensive equipment, airtightness management of the furnace, and the like. Moreover, although addition of Zn is performed for the purpose of securing the corrosion resistance of a product in the automotive heat exchangers and the like, the vacuum brazing is disadvantageous also in that Zn is not left in the material enough to secure sufficient corrosion resistance since Zn is evaporated under vacuum heating. Further, accumulation of the evaporated Mg or Zn on the furnace inner wall requires periodic cleaning of the furnace.
In response, recently, fluxless brazing under atmospheric pressure is proposed as a brazing method capable of solving the above-mentioned problems (refer to Patent Literatures 1 to 5).
For example, Patent Literature 1 proposes a fluxless brazing at atmospheric pressure in a non-oxidizing atmosphere by covering a brazing object while arranging an Mg-containing substance on the brazing object member or other portions.
Patent Literature 2 proposes a system such that a brazing object member is covered, inside a brazing furnace, by a windbreak (cover) which is preliminarily heated in the furnace to improve reduction in temperature rise rate.
On the other hand, as fluxless brazing that requires no cover, Patent Literature 3 proposes a method of adding Mg to brazing filler metal of a clad material, and fluxless-brazing the inside of a heat exchanger tube molded from the clad material at atmospheric pressure in an inert atmosphere.
Similarly, as a one that requires no cover, Patent Literature 4 proposes to enable an atmospheric brazing by adopting a laminated structure of a clad material, the clad material including an antioxidant layer clad on brazing filler metal surface.
Further, Patent Literature 5 proposes to enable a fluxless brazing under non-oxidizing atmosphere by cladding a brazing filler metal composed of an Al—Si—Mg alloy on a core material surface and pickling the material surface, prior to brazing, to reduce the thickness of an oxide film to 20 Å or less.